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Old 7th Oct 2018, 5:42 pm   #21
Mach One
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Default Re: Philips 531A Motorboating Problem (continued)

Yes, it has been discussed. The manual says 20,000Ω/v. I was led to expect in another post that the Fluke does offer a load and my manual says 10MΩ. If I remember my Avo days, there would have been a 250v range and 20,000Ω per volt would have come to a total of 5MΩ, so not so different and certainly not enough to make a bit difference across a 56kΩ resistor effectively?
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Old 7th Oct 2018, 5:59 pm   #22
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Default Re: Philips 531A Motorboating Problem (continued)

No it won’t will it.
If you short out the AGC line does the voltage drop?
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Old 7th Oct 2018, 8:41 pm   #23
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Default Re: Philips 531A Motorboating Problem (continued)

The cathode of V2 is returned to chassis through a switch, is the cathode at chassis potential? If that switch is high resistance it will reduce the current through the valve.
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Old 13th Oct 2018, 12:59 pm   #24
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Default Re: Philips 531A Motorboating Problem (continued)

Such good thinking - thank you!

After a busy week this morning is the first time that I have had an opportunity to have a play again. I have swapped over the reservoir capacitors and changed V1 (ECH42) for a beautifully clean and shiny NOS one. Now everything looks as it should - the voltage with no signal is in the mid-fifties! So, I suspect that the valve was low emission.

Now I have to check that it all works back in the case.

Just one more question: The permanent hum has disappeared but now there there is a hum on many of the stations when they are tuned in. It is not there when the aerial wire (actually a mains lead from another radio with the plug removed just for testing...) is removed. What might cause this? It sounds like a kind of modulation hum.
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Old 30th Jun 2019, 2:44 pm   #25
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Default Re: Philips 531A Motorboating Problem

Thanks to Paul who has allowed the original two "whacked threads" pertaining to the radio I have been refurbishing to be combined as they should have been originally, I can continue with the story of this set.

After another two months I have at last set-about trying to re-align it connected to my random wire antenna mounted in the garden. Due to the ever-present local interference conditions here (sounds like someone using one of those mains-borne Internet connection devices) reception on this wire is not as nice as with my Wellbrook ALA 1530 but the signals are very good.

However, two questions:

One is a repeat of the last one: What could be causing the hum on strong stations only (noticeable mostly on MW)? It is not there on another valve set that I have recently refurbished (the Philips 451A) so it is not a reception issue or the transmitters.

The other question is to do with the beehive trimmers that this set has. I have currently two other similar Philips radio chassis and each of those has the requisite 3 trimmers mentioned in the manual. This one has five! They do not appear to have been added subsequently but I do not know that. I will attempt to trace them to where they connect in the circuit but if anyone has anything to add about them I would be interested.

I have attached two pictures of the different sets.
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Old 1st Jul 2019, 7:39 am   #26
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Default Re: Philips 531A Motorboating Problem

It sounds like the phenomenon known as "modulation hum". Here are some tips for resolving it: https://www.vintage-radio.net/forum/...ead.php?t=2935 Can't help re: trimmers except if the set is otherwise working well, leave them alone! Cheers, Jerry
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Old 1st Jul 2019, 9:00 am   #27
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Default Re: Philips 531A Motorboating Problem

Thank you. I certainly suspect the areas mentioned in that thread. I'll have a play.

As for the other trimmers, I have had a go at following where they are connected to and it looks like they might be additional ones to improve the ability to accurately track the tuning of the antenna circuits on the three bands - otherwise there is really only one antenna tuning preset capacitor for all bands (C9).

However, I don't think that I have seen these extra ones on any of the other of this chassis that I have dealt with. As this one is one of the earlier ones I would guess (it has the earlier ECH42 rather than the [in my opinion] more common later ECH81) it surprises me that it was fitted then and not on subsequent ones. Another interesting Philips conundrum...
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Old 3rd Aug 2019, 6:57 pm   #28
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Default Re: Philips 531A Motorboating Problem

For those of you still interested, I may have solved the modulation hum problem.

I have another similar chassis (341A) which I have recently re-furbished and that does not exhibit the same problem. having checked out all the ht bypass capacitors on the 531A and finding no problem I wondered what the difference might be.

I had re-wired both radios three-core mains cable but the connection points for the earth points were different. On the 531A I had simply connected the mains earth to the chassis somewhere under the mains transformer on the right-hand side of the radio. On the 341A I had brought the mains earth cable out near the antenna / earth socket and connected it via a removable banana plug on the earth. So, I rewired the 531A to be identical to the 341A and that seems to have solved the problem, at least as far as I can tell for now. The actual eathing point is therefore of great importance - obviously.

Another interesting observation on the 531A, more an observation on reception conditions today generally, and certainly once again, obvious: Years ago a mains earth would have been quite adequate. Having installed a proper earth spike in the back garden and run an earth cable to the house for radio use, I had the opportunity to compare reception with the two earthing methods.

Unsurprisingly connecting the mains earth actually increased the background low-level noise of radio reception whereas the garden earth spike connection was quite a bit quieter. A separate earth therefore is a great advantage for good AM signal reception these days in particular.
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Old 16th Sep 2021, 11:57 am   #29
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Default Philips 531A Revisited

I say revisited - it was featured in this thread a couple of years ago

There was a question asked which was never answered and this is just in case anyone has any informatin to answer it.

Attached are two pictures:

One is of the set in question and the other is of the electrically similar 532A set. My 531A has FIVE beehive trimmers and the other set has THREE. Only the three are mentioned in the manual. Does anyone know anything about the extra two?
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